It may be because your connection is slower than it should be; most people will always use a separate stylesheet and there isn't usually much problem.
However, the ways to avoid this would be to put the styles inside head section style tags and not use a separate stylesheet, but I wouldn't do it.
Make your stylesheet as brief as possible.
As the link to a separate stylesheet is in the head section, I think that it is probably downloaded into computer memory before the browser parses the rest of the page; so it may be something in the html body section that is delaying the use of the styles which are already in memory.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.